THE MOLE
A mole of a substance is the amount of
that substance that contains the same number of elementary particles as there
are carbon atoms in 12.00000 grams of carbon-12.
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One mole of carbon-12 has a mass of
12.0g.
One mole of hydrogen atoms has a mass of
1.0g.
One mole of hydrogen molecules has a mass
of 2.0g.
One mole of sodium chloride has a mass of
58.5g.
The number of particles can be calculated
by multiplying the number of moles by Avogadro’s number. The number of moles
can be calculated by dividing the number of particles by Avogadro’s number.
The mass of one mole of a substance is known as its molar mass, and has units of gmol-1.
It must be distinguished from relative atomic/molecular/formula mass, which is
a ratio and hence has no units, although both have the same numerical value.
The symbol for molar mass of compounds or molecular elements is Mr.
The symbol for molar mass of atoms is Ar.
Mass (m), molar mass (Mr or Ar) and number of
moles (n) are thus related by the following equation:
Mass must be measured in grams and molar mass in gmol-1.
REACTING
MASSES
It is possible to use the relationship moles = mass/mr to deduce the masses of
reactants and products that will react with each other.
When performing calculations involving reacting masses, there are two
main points which must be taken into account:
The total combined mass of
the reactants must be the same as the total combined mass of the products. This is known
as the law of conservation of mass.
The ratio in which species
react corresponds to the number of moles, and not their mass. Masses must
therefore all be converted into moles, then compared to each other, then
converted back.
i) reactions which go to completion
Eg What mass of aluminium will be needed to react with 10 g of CuO, and
what mass of Al2O3 will be produced?
3CuO(s) + 2Al(s) à Al2O3(s) + 3Cu(s)
10 g
= 10/79.5
= 0.126 moles of CuO
3:2 ratio with Al
so 2/3 x 0.126 = 0.0839 moles of Al, so mass of Al = 0.0839 x 27 = 2.3
g
3:1 ratio with Al2O3
so 1/3 x 0.126 = 0.0419 moles of Al2O3, so mass
of Al2O3 = 0.0419 x 102 = 4.3 g
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